The present invention relates to a liquid-liquid extraction system. The present invention also relates to a process for using said system and the use of said system or process in removing aromatic compounds from organic streams, in treating an oil stream of a refinery, or in a liquid-liquid extraction process having at least two feed streams of different viscosity, similar density, or low interfacial tension.
Liquid-liquid extraction, which is also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, often water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid phase and is of utility, for example, in the work-up after a chemical reaction to isolate and purify the product(s) or in removing valuable or hazardous components from waste or byproduct streams in a variety or industrial processes. The extracted substances may be inorganic in nature such as metals or organic such as fine chemicals. Therefore liquid-liquid extraction finds wide applications including the production of fine organic compounds, the processing of perfumes, nuclear reprocessing, ore processing, the production of petrochemicals, and the production of vegetable oils and biodiesel, among many other industries. Certain specific applications include the recovery of aromatics, recovery of homogeneous catalysts, manufacture of penicillin, recovery of uranium and plutonium, lubricating oil extraction, phenol removal from aqueous wastewater, and the extraction of acids from aqueous streams.
In a typical industrial application, a process will use an extraction step in which solutes are transferred from an aqueous phase to an organic phase. Typically a subsequent scrubbing stage is used in which undesired solutes are removed from the organic phase, and then the desired solutes are removed from the organic phase in a stripping stage. The organic phase may then be treated to make it ready for use again, for example, by washing it to remove any degradation products or other undesirable contaminants.
Counter-current liquid-liquid extraction processes are particularly useful in obtaining high levels of mass transfer due to the maintenance of a slowly declining differential over the path of the counter-current flow. For example, industrial process towers generally make use of counter-current liquid extraction systems in which liquids flow continuously and counter-currently through one or more chambers or columns. The chambers or columns may have specially designed apparatuses mounted within them such as agitators for affecting the physical properties (e.g., droplet size) of the liquid and tower packing which serves to obstruct the direct flow of the liquids. Packing also provides for increased contact between lighter rising liquids and heavier settling liquids, and better contact means higher efficiency of the mass transfer process.
Liquid-liquid process towers and their columns are typically constructed to provide descending flow of a heavier liquid from an upper portion of the tower and ascending liquid flow of a lighter liquid from a lower portion of the tower. It is generally desirable to provide apparatuses and methods affording efficient mass transfer, or liquid-liquid contact, such that contact of the fluids can be accomplished with a minimum pressure drop through a given zone of minimum dimensions. Therefore high efficiency and low pressure drop are important design criteria in liquid-liquid extraction operations. Sufficient surface area for liquid-liquid contact is necessary for the reduction or elimination of heavy liquid entrainment present in the ascending lighter liquid. Most often, it is necessary for the structured packing array in the column to have sufficient surface area in both its horizontal and vertical plane so that fractions of the heavy constituents are conducted downwardly, and the lighter liquid is permitted to rise upwardly through the packing with minimum resistance. With such apparatuses, the heavy and light constituents of the feed are recovered at the bottom and top of the tower, respectively.
Counter-current liquid-liquid extraction columns may be passive or static packed columns. Static extraction columns typically rely completely on the packing/internals and fluid flow velocities past the internals to create turbulence and droplets. They offer the advantages of (1) availability in large diameters for very high production rates, (2) simple operation with no moving parts and associated seals, (3) requirement for control of only one operating interface, and (4) relatively small required footprint compared to mixer-settler equipment. High flows are typically required for obtaining adequate mass transfer though. Such passive columns suffer from limitations in that channeling may occur in which very little contact occurs between the liquids. Another problem is that generally only relatively few and large droplets of the first liquid phase are dispersed for relatively short periods of time in the second continuous liquid phase in passive columns. Thus relatively low degrees of mixing and thus reduced mass transfer and stage efficiency are associated with passive or static columns. As a result applications of static extraction columns are typically limited to those involving low viscosities (less than about 5 cP), low to moderate interfacial tensions (typically 3 to 20 dyn/cm equal to 0.003 to 0.02 N/m), low to moderate density differences between the phases, and no more than three to five equilibrium stages.
The low mass-transfer efficiency of a static extraction column, especially for systems with moderate to high interfacial tension or density differences, may be improved upon by mechanically agitating or pulsating the liquid-liquid dispersion within the column to better control drop size and population density (dispersed-phase holdup). Many different types of mechanically agitated extraction columns have been proposed. The more common types include various rotary-impeller columns, and the rotating-disk contactor or pulsed columns such as the reciprocating-plate column. In contrast to static extraction columns, agitated extraction columns are well-suited to systems with moderate to high interfacial tension and can handle moderate production rates.
Nonetheless it is important to provide just the right amount of mixing in agitated extraction columns. Higher agitation (more mixing) minimizes mass transfer resistance during extraction but contributes to the formation of small and difficult-to-settle droplets or emulsions and thus entrainment or “flooding” in the process. In designing a liquid-liquid extraction process, normally the goal is to generate an unstable dispersion that provides reasonably high interfacial area for good mass transfer during extraction and yet is easily broken to allow rapid liquid-liquid phase separation after extraction. Therefore over agitation may unfortunately require very long subsequent settling times in order to separate the phases.
The incorporation of agitator systems into passive static extraction columns in order to allow for the input of energy for increasing mixing is known from U.S. Pat. No. 2,493,265; U.S. Pat. No. 2,850,362; and WO 97/10886. Such agitated packed columns are characterized by a series of several alternating mixing and calming sections. The mixing sections have an agitator to promote intimate equilibrium contact between the liquids. The calming sections contain packing to stop the circular motion of the liquids and to facilitate their separation. Nonetheless such agitated packed columns according to the prior art are not well suited for systems that tend to emulsify easily owing to the high shear rate generated by a rotating impeller. In particular, the use of alternating mixing and calming sections means that any emulsions that are separated by a calming section will simply be regenerated by the subsequent mixing section in the series. Therefore the emulsions will be progressively built up by the high shear rates in each mixing section over the path of the column.
An additional problem is that many physical properties may change significantly with changes in chemical concentration during extraction. These properties may include interfacial tension, viscosities, and densities, and they strongly affect the mass transfer and thus extraction performance. In particular, changes in these properties promote problems with emulsion formation for a particular set of column conditions. Extraction processes involving high degrees of mass transfer are particularly susceptible to such changes in physical properties over the column length. One type of extraction column—static (passive) or agitated (active)—will not be able to deal well such systems and their property changes.
In such cases of changing physical properties, apparatuses may be used based on a combination of two or more different individual columns. Each column may have a different design and type of internals for optimum use with the specific physical properties at that particular stage of the extraction. Such apparatuses however require two individual column shells, two sets of feed pumps and two sets of process controllers. The process streams are processed by passing sequentially through these at least two columns. Such apparatuses based on a combination of individual columns have several disadvantages such as requiring a large number of auxiliaries such as pumps and piping, and elaborate process control means. Furthermore internals like distributors and/or collectors and phase separation will be necessary between each of the various columns of the apparatus.
The earlier discussed agitated packed columns of U.S. Pat. No. 2,493,265; U.S. Pat. No. 2,850,362; and WO 97/10886 are also not suited to extraction of systems involving high mass transfer and/or significant changes in physical properties due to changes in concentrations over the course of the extraction process and column. The disclosed columns are based on a substantially symmetrical arrangement of alternating mixing and calming sections over the column length, whereas the chemical concentration of the specie and physical property are asymmetrical over the extraction and will either increase or decrease along the column axis. Therefore the disclosed columns cannot take advantage of the particular suitability of a mixing versus a static section for a particular concentration and set of physical properties at the start versus the end of the extraction process (e.g. at the bottom versus the top or vice versa in the case of a substantially vertical column).
Alternatively, the use of cascades of combined single-stage mixer-settler tanks are known for use in extraction processes, for example, CN101219289(A) discloses such systems for the extraction of high flow rate solvent. Such cascade systems are not economical in cases where higher extraction efficiency is required because this will require a large number of mixer-settlers, and a large number of mixer settlers requires a large investment, as well as room space or “footprint”. An additional disadvantage of such cascades is that a control loop is needed for each of the devices.
US 2004/0222153 A1 discloses combinations of extraction and back extraction processes in which an extractor, such as a mixer settler, is combined with a back extractor in the recovery of 1,3-propanediol from an aqueous feed stream. However such systems do not address the problem of the extraction of liquid systems involving significant changes in physical properties while still offering adequate mass transfer efficiency and without a tendency to form emulsions or entrainment. Instead the systems of US '153 A1 address the problem of how to recover a diol from a solvent by back extraction after the diol is extracted to solvent from an aqueous stream by solvent in a first stage. Thus the flow patterns of the streams to the extractor and back extractor in US '153 A1 are quite different from those in multistage extraction systems.
Another alternative is to use centrifugal extractors, for example, as disclosed in U.S. Pat. No. 1,105,954 or U.S. Pat. No. 2,670,850. However by nature, such centrifugal machines are costly in terms of both investment and operation. They are additionally complex machines, and their high speed rotations require specialized mechanical devices and entail safety and noise concerns.
In conclusion, it would be desirable to have an liquid-liquid extraction system that would be better suited for extraction of liquid systems involving significant changes in physical properties than those of the prior art, and while still offering adequate mass transfer efficiency and without a tendency to form emulsions or entrainment and without requiring large investments, complex process controls, and extensive footprints.